It works great except it takes up a lot of table space. Do it.
You would just be using the DD as a belt drive in this case. I think the advantage of direct drive is that there is no slippage-in other words, the torque is sufficient to pull the record past the stylus friction for improved dynamics. Unless the SME has a very heavy platter that would act as a flywheel of sorts, I suspect the difference would be negligable. That being said, if you've got the time to monkey with it- try it!.
I used VHS tape to drive a passive platter with a DD table. Make sure you use a table with speed control. Models I recommend are Technics SL-M3, SL-M2 and JVC QL-F6, all have flat rim for the VHS tape to ride on. Other models will work too and you can also try thread drive. VHS is the only interface I tried so I vouch for others. You get the speed accuracy of DD but the decoupling of the tape filters out the cogging so you get "belt" drive table with the advantage of DD minus the sterile sound. The set up is a hassle at first, I have to say... but you get to realize how poor the motor is in many belt-drive designs. If you have the table space and time, once set up it's pretty stable. I don't have the space to do a set up like that anymore but it works. Have fun.
This would be a very interesting experiment. But, think about the bearing on the DD table. It probably was never intended to take the side load imparted upon it while driving another platter by belt. Only the downforce of a balanced platter spinning. I'm doubtful that you will actually improve the sound of the SME in this way, but, it would be fun. Maybe on a lesser table, the results could be an improvement.
One of the benefits of driving one platter with another is reduction of belt creep, and if you do it with a thread, there are fewer of the other issues associated with BD. Everyone who does this swears by it, but we all swear by all the things we either pay big bucks for or that require a lot of effort, as in this case. None of this is true scientific experimentation, but so what.
Just make sure the Denon is really operating up to its max. To do that, you might ought to change out all the old electrolytic capacitors in the Denon for new ones. And then, in the case of any Denon, you are going to have to do some cutting, I think, to enable your thread drive to encircle the Denon platter, since the edge of the platter is not so accessible, and the platter itself was never made to be encircled by a thread or belt (i.e., no groove).
Rube Goldberg rides again! As Harv says it will still be belt drive and the SME has a very good belt drive already. I have seen this done but usually using a much more massive belt driven platter as a flywheel to drive a second platter by a second belt. Too much trouble for me but if you feel like it try it.